*******Note that forecasts and outlooks in this post are NOT the official forecast from the National Hurricane Center (NHC). They are my own detailed views on the Atlantic tropics based on current observations and latest computer model runs. As such do not make decisions based on my posts...consult news media...watches and warnings from your local weather office...and any evacuation orders issued by local governments to make the most informed and best decisions. Visit the NHC website hurricanes.gov (hurricanes dot gov) for the latest watches/warnings and official forecasts on active tropical cyclones.**********
...WEDNESDAY OCTOBER 14 2020 12:39 PM EDT...
See area of interest sections below for a pair of tropical waves of low pressure being monitored for tropical development. Elsewhere in the coming days...three additional areas of interest for tropical development may emerge across the Atlantic as follows:
(1) The central North America upper trough has amplified further into a vortex under the cold air advection (southward cold air transport) on the west side of the surface frontal cyclone it generated over the last few days. A series of upper troughs will pivot around the south side of the upper vortex over the next several days...one of which is about to break away from the vortex and move into the open central Atlantic. It is possible this upper trough fragment triggers an open central Atlantic surface low pressure capable of acquiring tropical characteristics if it amplifies enough...and whether or not this happens depends on how much the current western Atlantic upper ridge and expected north Atlantic upper ridging amplifies in the warm sector of frontal systems to be supported by other upper troughs pivoting around the central North America upper vortex. So far the CMC model has led in showing a more amplified upper trough fragment and potential subtropical cyclone formation. While the GFS and ECMWF are now leaning to a more amplified upper trough fragment and better defined surface low pressure forming...those models are still not as impressive as the CMC while showing the upper trough fragment not being amplified enough such that it shifts eastward toward the current north Atlantic upper trough...placing the surface low under the less favorable western convergence zone of the upper trough fragment. Regardless of the upper air evolution...any open central Atlantic surface low pressure that does form will drift southwestward as the current northwest Atlantic surface ridge intensifies under the convergence zone of the expected north Atlantic upper ridging...and from surface ridges over eastern North America that will be supported by the western convergence zones of the other upper troughs to pivot northeastward around the central North America upper vortex. I have not added the possible central Atlantic surface low as an area of interest for tropical development as I am waiting for other models to agree with the upper air evolution shown in the CMC.
(2) A broad surface low pressure has formed in the open eastern tropical Atlantic under the divergence zone of an upper vortex that has coalesced in the region over the last several days. However the current surface low pressure is over waters too cool and the upper vortex is too warm to support the instability typically needed for thunderstorms and tropical development...and the surface low pressure is also too broad with no tight spin. Surface pressures will remain low in this region over the next several days as an impressive surface frontal cyclone develops in the vicinity of the Azores under the support of the north Atlantic upper trough as it amplifies into a northeast Atlatnic upper vortex...with the amplification of the upper trough to be kicked off by the amplification of north Atlantic upper ridging triggered by the warm sector of mutliple frontal systems/upper troughs pivoting around the central North America upper vortex. While the forecast northeastern Atlantic upper vortex will be much colder to support the development of thundestorms and possible tropical development...models disagree on how consolidated the surface low pressure field will be...with the GFS showing consoldiation and in the long range...and the NAVGEM showing consolidation and sooner. Will consider adding an area of interest for possible tropical development in the northeast Atlantic if the models come into better agreement on a consolidated surface low pressure or if observations yield a consoldiated surface low pressure in the coming days.
(3) By next week...a portion of the current western Atlantic to Gulf of Mexico upper ridge is forecast to shift southward into the western Caribbean Sea in response to the series of strong upper troughs to pivot around the central North America upper vortex. This will increase the upper outflow over the western Caribbean...potentially dropping surface pressures and increasing the coverage of thunderstorm activity as some of long range model runs suggest. This could result in the formation of a tropical cyclone in the western Caribbean region in the long range.
AREA OF INTEREST #1...The tropical wave of low pressure moving toward the Lesser Antilles from the central tropical Atlantic appears to be finally giving in to the wind shear being generated by the upper trough in the region...as its low pressure spin has become less defined and more diffuse at a location along 15N latitude to the northeast of Barabados. It could be argued that the thunderstorms sheared off to the northeast of the diffuse spin are supported by the eastern divergence zone of the upper trough. This system should continue tracking west-northwest toward a surface ridge weakness induced by the north Atlantic front and also the front driven by the remnant circulation of Delta moving into the region. This path will soon take the tropical wave toward the axis of the upper trough where there is less upper divergence to support thunderstorms...and even after passing through the upper trough axis the wave may have to deal with a zone of suppressive upper convergence between the west side of the upper trough and southeast side of the western Atlantic upper ridge. Thus I forecast a 0% chance of development. Not mentioning the Lesser Antilles in regards to this tropical wave on the home page bulletins of this site as the upper trough axis where there is a lack of divergence needed for thunderstorms remains settled over the islands. This is also my planned final statement on this tropical wave...unless it remains in the NHC tropical weather outlook tomorrow.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1200Z Oct 15)...0% chance of tropical cyclone formation (eastern Caribbean Sea just west of the Lesser Antilles near 16.5N-62W)
AREA OF INTEREST #2...The tropical wave of low pressure that has recently exited the west African coast and entered the eastern tropical Atlanic continues to have a broad spin near 12.5N-20W with bands of showers and thunderstorms on its north and west sides. Given that the broad spin has not angled northward so far in response to the low surface pressures in the northeast Atlantic...and that the current northwest Atlantic surface ridge will remain strong under the convergence zone of warm north Atlantic upper ridging at a location between the expected persistence of northeast Atlantic surface low pressure and another central Atlantic surface low pressure to form from the upper trough fragment soon to eject from the central North America upper vortex...I have nudged my forecast track points a bit southward. Given that the broad spin of the tropical wave has not shown signs of consolidation so far and that model support showing the tropical wave evolving into a surface low pressure has dropped...I have lowered my peak 5-day odds of tropical cyclone formation to 20%. For now the tropical wave remains protected from wind shear and has supportive upper outflow while under the eastern tropical Atlantic upper ridge axis. By late in the 5-day forecast period...wind shear is expected to increase once the upper ridge axis shifts southward due to the forecast amplification of the current north Atlantic upper trough into a cut-off upper vortex in the northeast Atlantic...as well as the push provided by the central Atlantic upper trough affecting area of interest #1 as well as the upper trough fragment about to eject from the central North America upper vortex as they both shift eastward with time. In response to the forecast increase in shear...odds are tapered down from the 20% peak to 0% by day 5.
The northern rain bands of this tropical wave will pass over the Republic of Cabo Verde Islands by later today and tomorrow...the home page bulletin on this site regarding this tropical wave reflects this.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1200Z Oct 15)...0% chance of tropical cyclone formation (south of the Republic of Cabo Verde Islands near 12.5N-25W)
IOH 48 Hr Outlook (1200Z Oct 16)...10% chance of tropical cyclone formation (eastern tropical Atlantic near 13N-30W)
IOH 72 Hr Outlook (1200Z Oct 17)...20% chance of tropical cyclone formation (eastern tropical Atlantic near 13N-35W)
IOH 96 Hr Outlook (1200Z Oct 18)...10% chance of tropical cyclone formation (central tropical Atlantic near 13.5N-40W)
IOH 120 Hr Outlook (1200Z Oct 19)...0% chance of tropical cyclone formation (central tropical Atlantic near 13.5N-45W)
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)
0000Z CMC Model Run....For areas of interest #1 and #2...no tropical cyclone formation shown. Elsewhere...upper trough fragment ejecting from central North America upper vortex amplifies in the open central Atlantic...generating a surface low pressure near 30N-55W at 66 hours...surface low drifts southwestward to 27N-59W by 120 hours while potentially acquiring tropical characteristics.
0000Z ECMWF Model Run....For areas of interest #1 and #2...no tropical cyclone formation shown. Elsewhere...upper trough fragment ejecting from central North America upper vortex amplifies in the open central Atlantic...generating a surface low pressure near 30N-54W at 72 hours...surface low drifts southwestward to 27N-58W by 120 hours while remaining weak.
0600Z GFS Model Run...For areas of interest #1 and #2...no tropical cyclone formation shown. Elsewhere...upper trough fragment ejecting from central North America upper vortex amplifies in the open central Atlantic...generating a surface low pressure near 29N-54W at 66 hours...surface low drifts southwestward to 26N-61W by 120 hours while remaining weak.
0600Z NAVGEM Model Run...For areas of interest #1 and #2...no tropical cyclone formation shown. Elsewhere...upper trough fragment ejecting from central North America upper vortex amplifies in the open central Atlantic...eventually generating a small surface low pressure near 29.5N-59.5W by 168 hours that acquires tropical characteristics in the long range. In the northeastern Atlantic...large and strong frontal cyclone develops in the northeast Atlantic over the Azores with the support of the current north Atlantic upper trough as the trough amplifies into a cut-off upper vortex...by 138 hours the frontal cyclone consolidates enough south of the Azores such that potential acquisition of tropical characterstics is possible with the support of the instability of the cut-off upper vortex cold temperatures.